Harley Hindered by Lower Belt Guard

As it was, I needed to re-adjust the belt tension and replace the lower belt guard (using some lock-tight blue on the replacement screw). The belt was not visibly damaged, the pulley had minimal damage -- something like two stone nicks. The belt tension, however, was pulled from the 6mm specified slack to around 80mm! I think I'm also going to get some of the aftermarket stainless mesh grills to keep stones out of the radiator cowl, and pay more attention to rattles.

On further investigation, I found there are some engineering shortcomings in that lower belt guard. It was designed such that one little screw could loosen from vibration and the entire guard will drop off its mounts. There are no sheet metal tabs against the hex flats, back-up cotter pin, clip, safety wire, or even a second screw through a closed slot.

If the belt guard did drop off, it would pass between the belt and pulley with an inverted-U channel. That would virtually guarantee that the guard would get trapped and pinched between the pulley and belt. This would, at the very least, either lock up the rear wheel with damage to the $99 guard, the $239 belt, and the $99 pulley (plus labor) or snap the belt and damage the pulley. Naturally, the possible catastrophic result is death or serious injury to the rider along with totaling the $17K+ bike.

It seems to me that a good Design FMEA should have caught this likely scenario from one little screw in the suspension/driveline loosening, and the risk could have been mitigated with secondary locks or travel limits.

Additionally, a redesign of the lower guard to wrap as a U channel from the outside of the belt would have allowed the guard to drop off. Or, alternatively, the channel riveted seam from metal to plastic could be shifted to have the top of the channel as plastic that may shear off without locking the rear wheel. In that alternative, it would need to be tested to make sure the wheel would not lock.

This entry was submitted by David T. Humphrey and edited by Rob Spiegel.

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That's why bikes used to use chains. Seriously, though, this column points to a common design flaw found in many products, where there's a single point of failure. And when I say point, I mean it literally, in that it often comes down to a single screw or fastener. Further, when that fastener comes loose (and usually not in the way it was intended; i.e., it comes out violently), you often get a stripped, ripped, or otherwise broken mounting point, which makes repair difficult.

Wow, that's one scary possible outcome from one very small design flaw. Just goes to show how important every component can be and why design for failure mode and effects analysis needs to be a critical part of the development workflow.

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